Dynamic mode decomposition analysis of the flow characteristics in a centrifugal compressor with vaned diffuser

Abstract

To understand the flow dynamic characteristics of a centrifugal compressor, the dynamic mode decomposition (DMD) method is introduced to decompose the complex three-dimensional flow field. Three operating conditions, peak efficiency (OP1), peak pressure ratio (OP2), and small mass flow rate (near stall, OP3) conditions, are analyzed. First, the physical interpretations of main dynamic modes at OP1 are identified. As a result, the dynamic structures captured by DMD method are closely associated with the flow characteristics. In detail, the BPF/2BPF (blade passing frequency) corresponds to the impeller–diffuser interaction, the rotor frequency (RF) represents the tip leakage flow (TLF) from leading edge, and the 4RF is related to the interaction among the downstream TLF, the secondary flow, and the wake vortex. Then, the evolution of the dynamic structures is discussed when the compressor mass flow rate consistently declines. In the impeller, the tip leakage vortex near leading edge gradually breaks down due to the high backpressure, resulting in multi-frequency vortices. The broken vortices further propagate downstream along streamwise direction and then interact with the flow structures of 4RF. As a result, the 8RF mode can be observed in the whole impeller, this mode is transformed from upstream RF and 4RF modes, respectively. On the other hand, the broken vortices show broadband peak spectrum, which is correlated to the stall inception. Therefore, the sudden boost of energy ratio of 14RF mode could be regarded as a type of earlier signal for compressor instability. In the diffuser, the flow structures are affected by the perturbation from the impeller. However, the flow in diffuser is more stable than that in impeller at OP1–OP3, since the leading modes are stable patterns of BPF/2BPF.

Spatial-Temporal evolution characteristics of pre-Stage-stall in a centrifugal compressor with a vaneless diffuser and a volute

“…6 The surge, which should be practically avoided, is usually triggered by the stage stall. A better understanding of the pre-stage stall evolution characteristic is needed to control the flow instability 7 and better evaluate the surge margin.…”

Time-delay-locked dynamic mode decomposition for extracting flow patterns of compressor in experiment

“…16 In recent years, the data of turbomachinery numerical simulation have been applied using DMD or POD method. [17][18][19] We have performed DMD with the data of numerical simulation as well, and unsteady flow structures in the tip region of axial compressor is well analyzed. 20 In this paper, the extension of DMD method in experiments allows for further identification of the characteristic correlations between the flow profile, the dominant vortex structure, and unsteady flow patterns.…”

Analysis on the unsteady flow structures in the tip region of axial compressor

“…[16][17][18] By using of the DMD method on the flow near the endwall of a low-pressure turbine, Romero and Gross 19 studied the evolution process of the vorticity modes and applied the modes on the designing of the active control system. Bases on plenty of the transient results from a centrifugal compressor, the effects of the unsteady characteristics under rotating frequency were well understood with the help of DMD method by Li et al 20 However, to the best of the author's knowledge, there is few published materials on applying the DMD mode to the tip region study of axial compressors.…”